Apparatus for reducing radio interference



Feb. 4, 1958 M. M. NEWMAN 2,822,465

APPARATUS FOR REDUCING RADIO INTERFERENCE Filed Oct. 26, 1953 li'rverflor Morris. M Newman United States Patent 2,822,466 APPARATUS FOR REDUCING RADIO INTERFERENCE Morris M. Newman, St. Paul, Minm, assiguor to Lighting & Transients Research Institute, Inc.,- Minneapolis, Minn., a corporation of Minnesota Application October 26, 1953, Serial No. 388,299 9 Claims. Cl. 250-20 polarity.

Some of the most'severe' interference to aircraft radio communications is due tocorona 'on the antenna itself,

which may be caused by high gradients from-selfcharging of the aircraft under precipitation-static conditions or by external thunder cloud fields. Usually the greatest improvements attainable in reducingsuch antenna corona interference are by measures to effectively reduce the gradients causing corona, such as for example by insulating the antenna wire. However, some corona interference will be obtained :even with the insulated wire until sutlicient charge F-is deposited, through a transient corona process, --to"1bring the "potential :of the insulating surface close enough tozthatof adjoining space so that .no further ionization results. In the case .of nearby lightning discharge the britennawin'sulation vis likely to be punctured and result in a particularly noisy antenna.- Also some antenna systems including insulatedwing .or .tailsections or emergency reeled .out traili'ng wires, are ditficult to insulate and are subject to corona interference. It =80 happens that tthe most. severe interfering corona transients which occur directly on the antenna are primarly .of one polarity. 'Other transients :of a single polarity may be producedby charged raindrops andcloselycoupled surface streamers. -It:is--an-object of the invention to provide-a constructicnbyimeans of which such radio interference-may be reduced priortofarrivaleat the receiver;

Another object of the invention resides .in providinga construction by means of which interference .caused by transient pulses of I either polarity may be-reduced.

A still further object aofthe invention resides in providing a construction in which radio .interference caused 1 by transient pulses ofieither polarity may be automatically reduced.

Another object of theinvention resides in providingan apparatus operatington' directcurrent disturbances oron the peaks of pulses.

' Other objects of-t-heinventionreside in.the.novel combination andarrangement'of parts and in the details of construction hereinafter. illustrated and/or described.

in the drawings: 1 Fig. lis awiringdiagranrof an embodiment of.the.iu-

vention.

Fig.2 is a diagrammatic view illustratinga number .of

transient. pulses of .a-si'ngle polarity.

Fig. 3 is a diagrammatic view .of the pulsestshow'n .in

" Fig. 2 combined with-a signal carrier.

Fig. 4 is a view 'sim'ilar to Fig.3 showing the, form ,of

the pulses and carrier after passing through the apparatus.

'In the drawings apparatus has been shown which includesa wide band amplifierfor amplifying the Signal and the noise, pulses anamount sfifiicient to operate the discrim'inating devices one of whichis ad'apte'd'tc blank put terminal of the wide 'band amplifier 12.

,.1o. apparatus. 5 The output. from the amplifier. isfedtto two.

ICC

out the positive pulses and the other the negative pulses. These devices include thermionic electron tubes which are fed through rectifiers,. one passing current of one polarity and the other of the opposite polarity. These tubes are biased to cause thetubesto normally conduct current and together function topass' the signal. Fed from the output'of the amplifier are two polarity sensing devices one of which is. energized by current of one polarity and the other by'current of theopposite polarity. These latter devices each produce a voltage'ofnegative polarity and the voltage produced when pulses of sufii: cient magnitude and number occur energizes either of these devices. Energization of either ofthese devices biases the discriminating tube with which the energized device isassociated to out 01f, thus causing the discriminating tube not, affected to pass: current of. a polarity opposite to that of the undesired pulses. The apparatus may also becansedto operate on; direct current transients.

In Fig. l the; invention is illustrated in diagrammatic form. Anan nna 13. 5 conventional form is connected to acondense 1-51 bymeans of. a condenser 152. The condenser 151 sgonnected by means of a conductor 14 .toa wide'bandamplifier 1 2 whichmay-be of any suitable construction such as is well-known in the art. This p fi henc eu v hc n n lea .f m in the-Wiring diagram, The output from the amplifier is fed by means of aconductor 15 to one contact 1530f a single pole d u e hro w ch 4,- .T e e he ce ac 0 this switch is connected by means "of a conductor 15.6 to a resistor 157 which inturnconnected. by means of a conductor 158 to the conductor 152. A. neon lamp 16 2 .is connected by means of a. conductor {163 to conductor 156 and by means of another conductor 164 to a common return. Another resistor 159 is. connected' tothe conductor 14 .bymeansof a conductor- 161 and to the common return. A stabilizingresistor 81 is connected by means of a conductor 824:0 conductor 15 and bymeans of a conductor 83 to the commonreturn. Where the noise pulses are of suificient strength .thewide band amplifier 12 may be dispensed. with.

The invention includes two discriminating devices 10 and 11. The device lt) comprises ,a thermionic tube 16 having a P t a ca ho .18, a heat 9 therefo a first control grid 20, a second control grid 21, a suppressor grid 22, and a screen grid 23. The cathode 18' is connected by means of a conductor 25 ,to the positive side of a crystal rectifier 26 The negative side of this rectifier is connected by means of a conductor 27 to a resistor28 which in turn is connected by means of a conductor .29 to the conductor 15 connected tothe out- A cathode resistor 48 connected to the common return and to the cathode 18 furnishes additional bias forth e tube'1 6. The

conductor 27 is further connected to the positive side of another crystal rectifier 31 by means of a conductor 32 and the negative side of this rectifier is connected by means of a conductor 33 to the common return. The

control grid20 of tube 16 is connected by means of a conductor 34 to an adjustable condenser '35 which in turn is connected by means of a conductor 36 to the conductor 27,. A grid leak 37 is connected" to the conductor 34 and "to the common return. The-suppressor grid *22'is connectedto the cathode -18 internallyof the tube.

Thediscriminating device ll is-similar to the device 10 and-comprises'a thermionic tube 40. This tube is identical withthe tube 16 and base plate 41, a cathode 42, a heater 43 therefor, a first control grid 44, a -second control grid 45, a suppressor grid 46 anda screen grid 47. The cathode 42 is connected by means of: a conductor '51 to the negative side of a crystal rectifier 5 2. The

positive side ofthis rectifieris connected by means of a Patented Feb, 4, 195.8

conductor 53 to-a resistor 54 which in turn is connected by means of a conductor 55 to the conductor 15. A cathode resistor 64 connected to the common return and to the cathode 42 furnishes additional bias for the tube 40. The conductor 53 is further connected to the minus side of another crystal rectifier 56 by means of a conductor 57. The positive side of this rectifier is connected to the common return through a conductor 58. The first control grid 44 of the tube 40 is connected by means of a conductor 59 to an adjustable condenser 61. This condenser is in turn connected through a conductor 62 to the conductor 53. A grid leak 63 is connected to the conductor 59 and to the common return. The suppressor grid 46 is connected to the cathode 42 internally of the tube.

The plates 17 and 41 of the tubes 16 and 40 are connected together by means of a conductor 65 which in turn is connected to a conductor 66. Conductor 66 is connected to a condenser 67 which in turn is connected to one contact 165 of a selector switch 166, the blade 167 of which is connected by a conductor 173 to the output terminal 68 and to which the receiver with which the apparatus is to be used may be connected. Voltage for the plates 17 and 41 is furnished from a B battery 69 which is connected to the common return and which is further connected by means of a conductor 71 to a resistor 72. Resistor 72 is in turn connected to the conductor 65 and thus to conductor 66 by means of a conductor 70. Voltage for the two screen grids 23 and 47 is furnished from the B battery 69. The two screen grids are connected together by means of a conductor 73 which in turn is connected to a conductor 74. Conductor 74 is connected to a dropping resistor 75 which in turn is connected by means of a conductor 76 to the conductor 71. A by-pass condenser 77 is connected between the conductor 73 and the common return through conductors 78 and 79.

Switch 166 has in all four contacts 165, 168, 169 and 171. Contact 165 is connected as previously described. Contact 168 is connected by means of a conductor 172 to conductor 15. Contact 169 is connected by means of a conductor 175 to conductor 25, while contact 171 is connected by means of a conductor 170 to conductor 51.

The tubes 16 and 40 are normally biased by virtue of the cathode resistors 48 and 64 and the grid leaks 37 and 63 to conduct current resulting from the signal impressed upon the same so that when there is no interference the receiver operates in a normal manner. To cause either of the tubes 16 or 40 to chop 011 the interfering pulses two polarity sensing devices 84 and 85 are employed. The polarity sensing device 84 comprises a thermionic tube 86 which has a plate 87, a grid 88, a cathode 89 and a heater 90 therefor. The cathode 89 is connected by means of a conductor 92 to the common return. The grid 88 is connected by means of a conductor 93 to thenegative side of a crystal rectifier 94. The positive side of this rectifier is connected by means of a conductor 95 to the blade 175 of switch 154. A grid leak 96 is connected by means of conductors 97 and 98 to the conductor 93 and to the common return. This grid leak is shunted by means of a condenser 99. The plate 87 of the tube is connected by means of a conductor 101 to a resistor 102 which is used for the purpose of loading the tube 86. This resistor is connected through a conductor 103 to a B battery 104 which in turn is connected by means of a conductor 105 to a resistor 106. Resistor 106 is connected by means of a conductor 107 to the common return. One side of a by-pass condenser 108 is connected to the conductor 105 and to the second control grid 45 of the tube 40 by means of a conductor 109.

The polarity sensing device 85 comprises two tubes 111 V and 112. Tube 111 consists of a plate 113, a grid 114,

a cathode 115 and a heater 116 therefor. Tube 112 in a similar manner includes a plate '117, agrid 118, a

cathode 119 and a heater 120 therefor. The cathode 115 is connected to the common return through a conductor 122. The grid 114 is connected by means of a conductor 123 to the positive side of a crystal rectifier 124. The negative side of this rectifier is in turn connected to the blade 175 of switch 154 through a con-- ductor 91. A grid leak 125 is connected between the conductor 123 and the common return and is shunted by a by-pass condenser 126. The plate 113 of the tube 111 is connected by means of a conductor 127 to one side of a load resistor 128. The other side of this load resistor is connected by means of a conductor 129 to a B battery 131. This B battery is in turn connected by means of a conductor 132 to a resistor 133. Resistor 133 is also connected through a conductor 134 to the common return. The resistor 133 is by-passed by a condenser 136 which is shunted about the same. The grid 118 of tube 112 is connected by means of a conductor to the conductor 132 associated with the B battery 133. The cathode 119 is grounded through a conductor 137. Plate 117 is connected by means ofa conductor 138 to a load resistor 139. This load resistor is in turn connected by means of a conductor 141 to the plus side of a B battery 142., The negative side of this B battery is connected by means of a conductor 143 to a resistor 144. Resistor 144 is connected by means of a conductor 145 to the common return. A conductor 146 is connected to the conductor 143 and to the second control grid 21 of the tube 16 of the discriminating device 11.

For the sake of simplifying the wiring diagram and making the invention more readily understood, the means for heating the filaments of the tubes has not been shown, though it can be readily comprehended that current will be passed through the filaments through conductors connected thereto and from suitable sources such as is now in common use and well-known in the art.

The operation of the invention is as follows: The switch 154 with switch arm to the left as shown in Fig. 1 provides simplest polarity sensing when the pulses remain of same polarity over relatively long periods such as is the case on an aircraft antenna passing different charged cloud areas. When polarity reversals occur, more frequently switch blade 154 can be moved to contact 153 to follow individual pulses or pulse groups. Switch 166 permits of diiferent kinds of performance. When blade 167 engages contact 168 the apparatus is completely cut out and the receiver operates in the customary, manner. When blade 167 engages contact 169, the apparatus operates to reject positive pulses, and when the blade contacts contact 171 it operates to reject negative pulses. When the blade engages contact 165, the sensing devices 84 and 85 become operative and the apparatus is sensitive to pulses of either polarity. Assume that the switch blade 175 of switch 154 has been moved to the right and that switch 166 is positioned as shown in Fig. 1. Condensers 35 and 61 are first adjusted to compensate for the stray capacities of the rectifiers. The tubes 16 and 40 are normally biased through the cathode, resistors and grid leaks thereof to pass signal current when no noise pulses are present and in the customary manner. Tubes 86, and 112 are, however, biased to pass current only when the voltage produced by the interefering pulses is considerably greater than the signal voltage. The time constant of the condenser resistor combinations 96-99 and 125-126 are such that the said condensers are quickly charged but are slow to discharge. The noise pulses, if positive, charge the condenser 99, and if negative charge the condenser 126. When a number of pulses of suflicient amplitude have been received to charge either of these condensers, the

tubes to which they are connected commence to operate and in the case of the tube 86 a negative voltage is impressed on the conductor 109 and thus on the second 'anumber of pulses 147 is shown;

duce'sufi'icient voltage to bia'sthe' tube 40' to cutofif This chops'ofi? one-half ofthe wave, namely, the half in which the pulses are contained. Iir'Fig; 2 a 'wave' form'ha'ving In Fig. 3, thecom bination of these pulses with a signalcarrier 1'48is' shown. The resultant clipping action' of the discriminating device 11 is-shown inFig; 4. Itwill be here'noted that the amplitude of the noise pulses has been greatly re ducedand in some cases reducedto zero. However, it has been found that asign'al with one-half of its lobes removed still functions fairly well and 'without too much distortion. Whatever residue of the noise pulses remains is of minor consequence and does not appreciably affect the signal. Thus the oifendin'g'p'ortio'ns of the noise pulses have been 'eliminatedmaking the 'signalmore tolerable and comprehensible. The operation of the polarity sensing device'85 is similar to that of'the polarity sensing device 84. In this case, however, the voltage passed by the rectifier 124 is negative and the resulting voltage from the conductor 135 is positive since the tube 111 changes the phase of-the voltage 180 degrees. By passing the currentfrom the -tube111' through -tube 112, the voltage on the -conductor-146 and impressed'upon the second'control grid zl of tube 16 is thus'negative and when the negative-noise pulses become sufiicient 'to actuate this polarity sensing device the tube 16 is biased to cut off and the negative portion of the wave clipped ofiin the same manner as described in conjunction with the discriminating device 11 and the polarity sensing device 84. It will thus be comprehended that pulses of either polarity when encountered for a suflicient period of time will maintain the respective circuits energized to cut olf all of the portion of the incoming input to the system in which the noise pulses are contained.

The advantages of the apparatus is apparent. The apparatus is extremely simple and inexpensive to build and eliminates a certain phase of noise which becomes extremely troublesome particularly when the receiver is installed in an airplane traveling in a zone having a charged cloud formation. With the invention, the elfective clip ping level of the polarity discrimination is zero as far as voltage is concerned. Thus the uni-polarity corona interference is eliminated over the entire range from zero voltage while the carrier potential may be as low as residual receiver noise and still have passed components after the rectification process. Cross-modulation from other carriers is considerably lower than with amplitude limiting processes. With the instant invention, the application of the apparatus to a receiver does not introduce other noise in the receiver. The entire apparatus is light in weight and can be built to occupy a very small space thus making it particularly useful in airplanes.

Changes in the specific form of the invention, as herein described, may be made within the scope of what is claimed without departing from the spirit of the invention.

Having described the invention, what is claimed as new and desired to be protected by Letters Patent is:

1. In apparatus for radio receiver noise reduction, an antenna, a discriminating device including a thermionic tube, an input circuit fed by said antenna and non-inductively coupled to said tube a resistor in said circuit; a rectifier in said circuit following said resistor and passing current of one polarity between said antenna and tube, a second rectifier connected to said input circuit at a locality between said first named rectifier and the resistor and to a common return, and passing current of the opposite polarity and an output circuit connected to said tube and to the receiver.

2. In apparatus for radio receiver noise reduction, an antenna, a discriminating device including a thermionic tube, an input circuit fed by said antenna and non-inductively coupled to said tube a resistor in said circuit; a crystal rectifier in said circuit following said resistor and passing current of one polarity between said antenna and seesaw tube, a second crystal rectifier'connected to said input circuit a't'alocalitybetween said first named crystal rectifier and the resistor'and toa common return, andp'assing current of the opposite polarity" and an output circuit connected to said tube and to the receiver.

3. In apparatus for radio receiver noise reduction, an antenna; a discriminating'd'ev'ice includin'ga thermionic tube, and input circuit fed bysaid antenna and non-inductively coupled to one element ofsaid tube a'resistor in said circuit; a rectifier insa'id circuit following said resistor and passing current of one polarity between said antenna and tube, a" second rectifier connected to said input'circuit at'alocality between said'first named rectifier and the'resistor and to a common return, and passing current of the opposite polarity, a condenser connected to another element of said tube'and to said input circuitat a locality'b'etwe'en said first named rectifier and there'si'sto'r'and'an output circuitconnected to another element of saidtu'be and to the receiver.

4. In apparatusfor'radio receiver noisereduction, an antenna, a dis'criminatingdevice including a thermionic tubeliav'ing a'c'athode, a grid and a plate, an input circuit' fed bysaidahtenna' and' non-inductively coupled to'tliec'athode of said tube, a'resistor in said circuit a re'ctifi'er'ins'aid circuit'following-said resistor and passing'current of one polarity between said antenna and tube, a" second rectifier connected to' said-inputcircuit at a'loc'a'lity between'saidfii-s't'named' rectifier'an'd the resistor and to a common return, and passing current of the opposite polarity, a condenser connected to the grid of said tube and to said input circuit at a locality between said first named rectifier and the antenna and an output circuit connected to the plate of said tube and to said receiver.

5. In apparatus for radio receiver noise reduction, an antenna, a discriminating device including a thermionic tube, an input circuit fed by said antenna and non-inductively coupled to said tube a resistor in said circuit; a rectifier in said circuit following said resistor and passing current of one polarity between said antenna and tube, a second rectifier connected to said input circuit at a locality between said first named rectifier and the resistor and to a common return, and passing current of the opposite polarity, a second discriminating device similar to said first discriminating device and including two rectifiers arranged in reverse order with respect to the rectifiers of said first discriminating device and polarity sensing means connected to the antenna and to the tubes of said discriminating device and controlling the functioning of the tubes of said discriminating devices to eliminate the negative lobes of the input to the receiver when the noise pulses are negative and to clip off the positive lobes of the input to the receiver when the noise pulses are positive.

6. In apparatus for radio receiver noise reduction, an antenna, a wide band amplifier connected to said antenna, a discriminating device including a thermionic tube, an input circuit fed by said amplifier and non-inductively coupled to said tube and to said amplifier, a resistor in said circuit; a rectifier in said circuit following said resistor and passing current of one polarity between said amplifier and tube, a second rectifier connected to said input circuit at a locality between said first named rectifier and said amplifier and passing current of the opposite polarity and an output circuit connected to said tube and to the receiver.

7. In apparatus for radio receiver noise reduction, an antenna, a discriminating device, an input circuit fed by said antenna and non-inductively coupled to said discriminating device, said discriminating device including rectifying means and operating to pass current of one polarity, a second discriminating device similar to said first discriminating device and connected to said resistor and to the receiver, said second discriminating device passing current of the opposite polarity and polarity sensing means connected to the antenna and to said discrimi- '7 nating devices, and rendering one or the other of said discriminating devices inoperative when the noise pulses reaching said sensing means are of'the same polarity as the polarity of the lobes eliminated by the tube rendered inoperative.

8. In apparatus for radio receiver noise reduction, an antenna, a discriminating device, an input circuit fed by said antenna and non-inductively coupled to said discriminating device, said discriminating'device including rectifying means and operating to pass current of one polarity, a second discriminating device similar to said first discriminating device and connected to said antenna and to the receiver, said second discriminating device passing current of the opposite polarity and means for rendering either of said discriminating devices inoperative.

9. In apparatus for radio receiver noise reduction, an antenna, a discriminating device including a thermionic tube having a cathode, a first control grid, a second control grid, and a plate, an input circuit fed by said amplifier and non-inductively coupled to the cathode of said tube a resistor in said circuit, a rectifier in said circuit following said resistor and passing current of one polarity between said antenna and tube, a second rectifier connected to said input circuit at a locality between said first named rectifier and the antenna and to a common return and passing current of the opposite polarity, a second discriminating device similar to said first discriminating device and including two rectifiers arranged in reverse order with respect to the rectifiers of said first dlSCl'imie nating device, a polarity sensing device including a tube, an input circuit fed by said antenna and non-inductively coupled to said tube, a rectifier in'said input circuit, and passingcurrent of one polarity, an output circuit connected to said tube and to the second control grid of the tube of said discriminating device whose first named rectifier passes current of the same polarity as the rectifier of said sensing device, said sensing device being actuated by noise pulses of one polarity to produce a negative potential sufficient to bias said tube of the denoted discriminating device to cut oif, and a second sensing device similar to said first sensing device and connected to the antenna and to the second control grid of the other discriminating device and being actuated by the noise pulses of opposite polarity to produce a negative potential sufiicient to bias the tube of the other discriminating device to cut oli.

References Cited in the file of this patent UNITED STATES PATENTS 1,477,017 Sprague Dec. 11, 1923 1,711,658 Sprague May 7, 1929 1,789,208 Woodrufi Jan. 13, 1931 2,248,793 Terry July 8, 1941 2,434,929 Holland et a1. Jan. 27, 1948 FOREIGN PATENTS 145,023 Australia Feb. 7, 1952 

